Spaceflight is hideously expensive, roughly $10,000 per pound to Low Earth Orbit. And a large part of that is because the rockets that boost payloads into orbit are expended. Every rocket motor is an incredibly precise, extremely complicated engineering marvel. And yet, they’ve traditionally been used once, and thrown away.

Probably something like 90% of the fuel and thrust of a rocket sending a payload to orbit is spent sending the fuel and rocket up, not the payload itself. As any airline pilot can tell you, it takes fuel to haul fuel. That’s why most rockets are multistage. After burning the first stage, it’s just dead weight, and no sense hauling it any further. A lighter second stage with a smaller motor can take over.

But it is those very same first stage engines that are most expensive.

So Space-X looked at ways to recover those very expensive engines. And decided the best way to save them was to have the first stage make a controlled descent to the earth, eventually with the rocket landing on deployed landing legs. If that seems pretty incredible to you, well, you’re not alone. When I first heard of the plan, I was skeptical. It’s a difficult flight to control, and the extra weight of fuel needed imposes its own penalty.

But then, unlike my co-author Roamy, I’m not a rocket scientist.

Space-X first decided to see if they could actually control a rocket in low altitude and have it successfully land on its own feet, as it were. To do so, they built a low altitude rocket resembling the Falcon 9 first stage and called it the Grasshopper.

As for controlling the first stage after an actual launch, I forgot they would be letting the atmosphere do a lot of the work. When a rocket first takes off, it’s at its greatest weight, and in the thickest air, and so has the least acceleration. As the weight of fuel burns off, and the air resistance diminishes at altitude, and yet the thrust generated remains the same, the acceleration increases, reaching its maximum at burnout, or “staging” if you will.

So now our first stage, at something like 50 miles altitude, effectively the edge of space, is flying separate from the second stage and the payload. Rather than just tumbling down, it can use only one of its 9 motors to begin a controlled deceleration. And as it encounters ever thickening atmosphere, its speed will decrease at an ever increasing rate. The rocket itself is pretty light. A vast percentage of its takeoff weight is its fuel (and oxidizer, of course). With most of it gone, it takes less thrust (and consequently, even less fuel) to decelerate.

Friday’s launch was a test primarily of the ability of Falcon 9 to handle the high altitude part of a reusable booster, that is, the part from Mach 10 down to low airspeeds. And it was intended to drop the booster in the ocean. If things had gone wrong, slamming a rocket into the roof of granny’s house would be bad. Because it landed in a salt water environment, rebuilding the liquid fuel motors would be quite difficult. In the future Space-X hopes to land first stages on dry land. If they can successfully do so (and it is really starting to look like they can) and they can quickly refurbish the booster for a second flight, they may cut costs of launch, in terms of pounds to LEO in half. That would the most significant decrease in launch costs in the history of spaceflight.

The problem with DC-X was that it was SSTO. All that weight hauled up had to be brought back down. It’s a heck of a lot easier to decelerate the 1st stage F9R from 50miles/Mach 10 than it would be to decelerate the entire DC-X from 175m. Much less heat shielding, and much less thrust and fuel required (which means a greater percentage of the pad weight can be payload- which means better economy).

As it is, F9R expends 1 Merlin engine in the second stage and yet brings back 9 of them (that’s the “9” in the Falcon 9 booster) at a much lower techonological risk.

To pick nits, the DC-X was a proof of concept prototype not intended to reach orbit. They wanted to test controls, software, reusability, etc. But, yes, the final product was intended as SSTO.

And, yes, I’m excited about this. I can’t wait to see how they manage with repeated launches with short intervals in-between. The Clipper showed promise there. That’s when it becomes a consumer transport, instead of an esoteric launch vehicle.

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